This invention relates to a battery pack, a method for charging/discharging counting and a method for setting the residual capacity of the battery pack.
Up to now, there has been furnished a battery pack having battery cells as secondary batteries, such as lithium ion cells, NiCd cells or nickel hydrogen cells.
The battery pack usually includes a micro-computer for performing calculations of the residual battery capacity of the battery cells or communication with electronic equipment having the battery cell as a power source, peripherals of the micro-computer and a battery cell status detection circuit necessary in carrying out calculations of the residual battery capacity.
The number of charging/discharging cycles of the battery pack is not infinite. On the other hand, the maximum number of charging/discharging cycles, capable of maintaining practically tolerable charging/discharging characteristics, is determined to some extent depending on the type of the battery cell.
In the conventional battery pack, it has been difficult for the user to recognize the maximum number of charging/discharging cycles, capable of maintaining practically tolerable charging/discharging characteristics, that is the service life of the battery cell. The user is only able to recognize the approaching end of service life of the battery cell, as the charging/discharging is repeated, on recognizing that battery capacity is decreasing at a faster rate even though the cell has been charged to its full capacity.
In order to permit the user to recognize the service life of the battery cell easily, the present Applicant has proposed in Japanese Laying-Open Patent H-9-243718 a battery pack and a method for displaying the state of the battery, referred to below as the first disclosed technique.
The first disclosed technique, shown in FIG. 1, detects that the voltage of the battery cell has exceeded the first threshold voltage, and further detects that the voltage has become lower than a second threshold voltage. The first disclosed technique counts the number of charging/discharging cycles on the assumption that one cycle of charging or discharging occurs when one of the voltage states is detected after detecting the other voltage state.
On the other hand, in a battery pack having plural battery cells, the maximum charging voltage differs from one battery cell to another. Thus, the present Assignee proposed in Japanese Laying-Open Patent H-9-285026 a battery charging method and apparatus and a battery pack, referred to below as the second disclosed technique.
In the second disclosed technique, the residual battery capacity is computed based on the charging of the battery cell and the initial value (e.g., voltage) stored in the battery cell.
The battery pack is loaded on an electronic equipment, such as a video camera device, to furnish the current to the equipment. If the power source has become depleted, the battery pack is charged. This battery pack has a battery cell, as a chargeable and dischargeable secondary battery, and a micro-computer for detecting the voltage of the battery cell to compute the residual battery capacity.
Since the residual battery capacity is varied appreciably with temperature, it is computed using a temperature dependent coefficient of the battery cell, as stated for example in Japanese Laying-Open Patent H-9-297166. This temperature dependent residual capacity correction coefficient is referred to below as the correction coefficient. Since this correction coefficient differs in magnitude with temperature, the correction coefficient for each increment of 10xc2x0 C. is stored in a nonvolatile memory, such as ROM. The micro-computer computes the residual battery capacity using the correction coefficient stored in the non-volatile memory and which corresponds to the current temperature.
However, in the first disclosed technique, count-up is made only when the battery cell voltage falls below the second threshold value, such that, if the battery cell is charged before the cell voltage falls below the second threshold value, count-up is not made. Thus, there is raised a problem that, even though the battery cell is deteriorated due to charging/discharging, the cycle is not counted correspondingly. Although it may be attempted to set the second threshold to a higher value, count-up then occurs while as yet there is left sufficient battery power, such that count-up cannot be made correctly.
On the other hand, the conventional battery pack is designed so that, once the charging is made up to 90%, this 90% charging is deemed to be full charging, in order to absorb errors in current detection etc in charging. Thus, in the battery pack, the integrated value of the residual battery capacity on 90% charging is previously stored in the ROM and, if it is verified that charging up to 90% has been made, the battery capacity is set as the integrated residual battery capacity value.
However, if the charging/discharging is repeated, the battery cell is deteriorated, such that the battery capacity that can be retrieved actually is decreased. With this battery cell deterioration, the integrated residual battery capacity on 90% charging is lowered, thus producing a gap between the integrated residual battery capacity on 90% charging stored in the non-volatile memory and the actual integrated residual battery capacity.
If the residual battery capacity is computed based on the initial value stored in the battery cell at the time of charging, in accordance with the second disclosed technique, there is presented a problem that the residual battery capacity cannot be computed correctly.
It is therefore an object of the present invention to provide a battery pack in which, if a battery cell is deteriorated due to charging/discharging, the number of cycles is counted depending on the deterioration, and in which the residual battery capacity can be accurately set depending on the deterioration.
It is another object of the present invention to provide a method for counting the number of charging/discharging cycles in the battery pack, and a method for setting the residual battery capacity of the battery pack.
In one aspect, the present invention provides a battery pack including voltage detection means for detecting the voltage of a battery cell, storage means for storing a correction coefficient for calculating the residual battery capacity for a pre-set temperature, temperature detection means for detecting the temperature of the battery cell and computing means. This computing means operates so that, if a correction coefficient associated with a temperature detected by the temperature detection means is stored in the storage means, the computer means reads out the correction coefficient to compute the residual battery capacity based on the correction coefficient and the voltage detected by the voltage detection means. The computing means also operates so that, if a correction coefficient associated with a temperature detected by the temperature detection means is not stored in the storage means, the computer means reads out a correction coefficient associated with a pre-set temperature ahead and at back of the detected temperature from the storage means to compute a correction coefficient associated with the detected temperature based on each readout correction coefficient to compute the residual battery capacity based on the computed correction coefficient and the voltage detected by the voltage detection means.
In another aspect, the present invention provides a method for computing the residual battery capacity of a battery pack associated with a pre-set temperature including storing a correction coefficient for calculating the residual battery capacity for a pre-set temperature, and detecting the temperature of the battery cell, wherein, if a correction coefficient associated with a temperature detected by the temperature detection means is stored in the storage means, the correction coefficient is read out and the residual battery capacity is computed based on the correction coefficient and the voltage detected by the voltage detection means. Also, if a correction coefficient associated with the detected temperature is not stored in the storage means, a correction coefficient associated with a pre-set temperature ahead and at back of the detected temperature is read out from the storage means to compute a correction coefficient associated with the detected temperature based on each read-out correction coefficient to compute the residual battery capacity based on the computed correction coefficient and the voltage detected by the voltage detection means.